Future demands for high-end CT (computer tomography) and CV (cardio vascular) imaging regarding the X-ray source may be higher power/tube current, shorter response-times regarding the tube current, especially when pulse modulation is desired, and smaller focus spots corresponding to the demands of future detector systems.
One key to reach higher power in smaller focus spots may be given by using a sophisticated electron-optical concept. But of the same importance may be the electron source itself and the starting conditions of the electrons. For a thermionic electron emitter for X-ray tubes it may be essential to heat up a metal surface to get electron emission currents of up to 1-2 A. These electron currents within the tube may be necessary for state-of-the-art medical applications. For today's high-end X-ray tubes, directly or indirectly heated thin flat emitters are usually used.
FIG. 1a shows an example of a conventional directly heated thin flat emitter 101 having a rectangular outline. The flat electron emission surface 103 is structured with narrow slits 109 to define an electrical path and to obtain the required high electrical resistance. The thin emitter film is fixed at connection points 105 to terminals 107 through which an external voltage can be applied to the structured emission surface in order to induce a heating current for heating the emission surface to temperatures for thermionic electron emission, e.g. more than 2000° C.
This emitter concept may have small thermal response times due to its small thickness of 100 to 200 μm and sufficient optical qualities owing to its flatness. Variations of this design concept are implemented in today's state-of-the-art X-ray tubes.
FIG. 1b shows another example of a conventional directly heated thin flat emitter 201 having a circular outline. The flat electron emission surface 203 is structured with circularly curved narrow slits 209 to define an electrical path. Through connection points 205 and terminals 207 connected thereto, an external voltage can be applied to the emission surface for inducing a heating current.
FIG. 2 shows a schematic top view on an emitter 1 as shown in FIG. 1a. Slits 9 (the width of which is shown exaggerated in FIG. 2) are formed in the emission surface 3 such that a meander-like structure with a conduction path 11 results.
In order to achieve the level of electron emission necessary for example for application of the electron emitter in an X-ray tube, the above emitters described with respect to FIGS. 1a, 1b and 2 having a meander-like structured emission surface may be heated up to 2400° C. in their emission surface 3 by application of an electric current. Bordering surfaces 5 adjacent to the actual electron emission surface are also heated but the temperatures reached there are to low for thermionic electron emission. At elevated temperatures, the mechanical stability and rigidity of the emitter structure can be reduced significantly.
Due to its inertia, the electron emitter may experience accelerations of more than 30 g, e.g. caused by rotation of the emitter on a CT gantry. As a result of the application of such external load, the meander-like structure may deform in such a way that the width of the slits 9, 109, 209 in partial areas of the emitter increases and, more crucial, decreases in other partial areas.
Regardless of the direction of the applied external load, the highest maximum of mechanical stress is usually achieved in an area 13 of high curvature of the meander-like emitter structure as schematically shown in the enlarged partial view of FIG. 3b. In the figures, the external force F may be applied in any direction parallel to the surface of the electron emitter whereas the main mechanical stress loads L in the area 13 is usually directed along the X-axis as depicted in the figures.
The combination of the high temperature and the mechanical stress may lead to creep deformation of the emitter structure especially in the mainly loaded areas 13. Creep deformation in X-direction in such area can cause a pre-mature contact of the bars 12 forming the conduction path 11 of the meander-like emitter structure and, subsequently, may lead to a short circuit. This may deteriorate the electron emission characteristics of the emitter and, furthermore, may reduce the lifetime of the electron emitter.
There may be a need for an improved thermionic electron emitter and an X-ray source including same as well as for a method for preparing a thermionic electron emitter, wherein the electron emission characteristics are improved and/or the stability of such electron emitter characteristics over time is increased and/or the lifetime of the electron emitter is increased.